Japanese Patent No. 2,555,140 (Document 1) describes a method for extracting a timing component in order to demodulate a digital modulated signal of a PSK system, a QAM system or the like and reproduce information contained in the signal. In this method, a frequency component fs is extracted by nonlinearly processing a frequency component fs/2 (i.e., a frequency component which is ½ times a symbol rate fs) which is present in a digital modulated signal as symbol data changes.
Since this extraction method uses nonlinear processing, timing extraction can be stably conducted without being affected by a carrier frequency offset produced by a frequency error of a local oscillator used in a receiver. In order to extract a frequency component fs, digital signal processing must be conducted at a sampling frequency higher than 2fs according to the sampling theorem. A sampling frequency of 4fs or more is usually used in order to prevent interference.
Japanese Patent Laid-Open Publication No. 5-207082 (Document 2) describes another method for extracting a timing component. In this method is extracted a frequency component fs/2 which is present in a digital modulated signal as symbol data changes. The extracted frequency component is then subjected to vector processing, frequency shifting, and double angling. Accordingly, timing extraction can be conducted at a sampling frequency 2fs.
Japanese Patent Laid-Open Publication No. 7-226781 (Document 3) describes still another method for extracting a timing component. In this method, a frequency component fs is extracted at a sampling frequency 2fs by nonlinearly processing an averaged digital signal and obtaining the difference between the nonlinearly processed signal and a one-sample delayed version of that signal.
Problems
The method of Document 1 requires digital signal processing at a sampling frequency 4fs for stable timing extraction. This results in difficulty in hardware implementation and increased power consumption if the symbol rate is high. Even at a low symbol rate, this results in increased amount of processing per unit time if a DSP is used.
The method of Document 2 is affected by a carrier frequency offset produced by a frequency error of a local oscillator used in a receiver, because the method uses vector processing. Such a carrier frequency offset prevents accurate timing extraction.
The extraction method of Document 3 uses a sampling frequency 2fs. An extracted signal therefore causes interference according to the sampling theorem, thereby preventing stable timing extraction.